Like other toxic metals, such as chromium and uranium, zinc undergoes many chemical transformations during its journey from the source to the final resting place. The Imperial Smelting Process used for many decades in Europe produced considerable amounts of dust and fumes rich in zinc and lead. When the zinc reached the soil, it was deposited in the form of a number of "primary" zinc-containing minerals. Weathering of these minerals resulted in the trapping of zinc in "secondary" minerals, predominantly clay minerals but to a lesser extent iron oxyhydroxides and manganese oxides that immobilize the zinc, rendering it less harmful.

In recent years, x-ray diffraction and EXAFS of powder samples have been replacing older chemical-extraction techniques for speciation of contaminants in the environment. Making EXAFS measurements for a large number of compounds in well-prepared forms results in a library of standards that can be used when a sample contains multiple chemical compounds containing the same element. Mathematically fitting a combination of the EXAFS spectra from standards with the measured spectrum in principle identifies and gives the concentration of each compound in the sample. However, the method does not work well when too many standards are required, so it is necessary to have some idea of what is present, a difficult task when some compounds exist in small concentrations against a larger background of other compounds.

Molecular Environmental Science

As the industrialized nations of the world struggle to clean up polluted military, industrial, and agricultural sites, a new field of research has arisen that offers a scientific approach to waste remediation --molecular environmental science (MES). The transport of pollutants, such as heavy metals, from their source through soil and water to their final resting place and their impact on plants, animals, and humans along the way depends in detail on the chemical form of the polluting species. MES researchers aim to study at the molecular level the forms pollutants take in the environment, the processes by which they move through it, and the mechanisms by which they change from one form to another. In this study, a group of French researchers examined soils contaminated by zinc from decommissioned smelters. By combining new x-ray analysis techniques, they were able to identify the three main chemical forms taken by zinc in these soils and determine their relative proportions.

The French team attacked this problem for its soil samples by exploiting the newer third-generation synchrotron-radiation sources. Powder EXAFS hinted that zinc-containing clay minerals were the predominant species. Because of the layered structure of clays, EXAFS spectra with linearly polarized light has a distinctive angular dependence as an oriented sample is rotated in the beam, and this is what the team observed at the ESRF. A detailed analysis of the spectra revealed the local structure around the zinc in the clay.

Micro-EXAFS of zinc in a manganese-rich area (red) of a soil sample identifies the presence of zinc-sorbed birnessite (blue), a layered manganese compound with zinc above and below vacant sites, as shown in the model.

The two next most important zinc species (i.e., the iron oxyhydroxides and manganese oxides) were identified at the ALS. First, with µ-XRF, the team located regions containing manganese, iron, and zinc in these inhomogeneous samples. In particular, they found iron-rich grains that typically were 10 to 20 µm in size and manganese-rich spherules much larger at 300 µm. Application of µ-EXAFS within these regions then revealed the identity and structure of the zinc species. For example, the zinc-containing manganese compound was birnessite, a layered compound in which the zinc was adsorbed in the interlayer space above and below vacant sites with either four- or six-fold coordination. Model compounds derived from polarized and µ-EXAFS data were then used to obtain good fits of powder-EXAFS data for all the soil samples studied.

Model compounds derived from polarized-EXAFS data at the ESRF and µ-EXAFS data at the ALS provide a good fit with powder-EXAFS spectra from a soil sample.

Research conducted by A. Manceau, B. Lanson, M. L. Schlegel, J.-C. Hargé, M. Musso, and J.-L. Hazemann (University J. Fourier and CNRS, France); D. Chateigner (University of Maine-Le Mans, France) and G. M. Lamble (ALS).

Funding: DGAD/SRAE Division of the French Ministry of Environment (MATE) and U. S. Department of Energy, Office of Basic Energy Sciences.

Publication about this experiment: A. Manceau et al., "Quantitative Zn speciation in smelter-contaminated soils by EXAFS spectroscopy," Am. J. Sci. 300, 289 (2000).